a) Field of the Invention
The present invention relates to a tunnel junction structure, its manufacture method, and a magnetic sensor having such a tunnel junction structure.
b) Description of the Related Art
In a laminated structure of a metal layer, an insulating layer, and a metal layer stacked in this order, tunnel current flows as voltage is applied across the metal layers on both sides, if the thickness of the insulating layer is sufficiently thin (several angstroms to several tens angstroms). Such a junction is called a tunnel junction. A metal oxide film is generally used as the insulating film. For example, the surface layer of an aluminium layer is oxidized through natural oxidation, plasma oxidation, thermal oxidation or the like to form a thin film of aluminum oxide. By controlling the oxidation conditions, the thickness of the aluminum oxide thin film can be made several angstroms to several tens angstroms.
A tunnel junction having the metal layers on both sides made of ferromagnetic material, is called a ferromagnetic tunnel junction. A tunneling probability (tunnel resistance) of the ferromagnetic tunnel junction depends on the magnetization states of the magnetic layers on both sides. Therefore, if the magnetization states of the magnetic layers are changed by applying an external magnetic field, the tunnel resistance can be controlled. In other words, a change in the external magnetic field can be detected from a change in the tunnel resistance.
A tunnel resistance R is given by the following equation: EQU R=Rs+(1/2).DELTA.R(1-cos .theta.) (1)
where .theta. is an angle between magnetization directions in the magnetic layers on both sides. Rs is a tunnel resistance at the angle .theta. of 0.degree., i.e., when the magnetization directions in both the magnetic layers are parallel to each other. .DELTA.R is a difference between a tunnel resistance when the magnetization directions in both the magnetic layers are parallel to each other and a tunnel resistance when they are reversed-parallel to each other.
As seen from the equation (1), the tunnel resistance R becomes minimum when the magnetization directions in both the magnetic layers are parallel to each other, and becomes maximum when they are reversed-parallel to each other. This results from polarization of spins of electrons in the ferromagnetic material. Generally, an electron takes either an up-spin state or a down-spin state. An electron in the up-spin state is called an up-spin electron, whereas an electron in the down-spin state is called a down-spin electron.
In non-magnetic material, generally the number of up-spin electrons is equal to the number of down-spin electrons. Therefore, there is no magnetization in the non-magnetic material as a whole. In contrast, in ferromagnetic material, the numbers of up-spin and down-spin electrons are different. Therefore, there is up or down-direction magnetization in the ferromagnetic material as a whole.
It is known that each electron is moved by the tunneling phenomenon, while retaining its spin state. In this case, although an electron can be tunneled if there is an empty energy level corresponding to the spin state of the electron to be tunneled in the destination magnetic layer, it cannot be tunneled if there is no such an empty energy level.
A change rate .DELTA.R/Rs of the tunnel resistance is given by a product of a polarization factor of an electron source and that of a destination empty energy level, as in the following: EQU .DELTA.R/Rs=2P.sub.1 P.sub.2 /(1-P.sub.1 P.sub.2) (2)
where P.sub.1 is a spin polarization factor of an electron of an electron source, and P.sub.2 is a spin polarization factor of an empty energy level of a tunnel destination magnetic layer. P.sub.1 and P.sub.2 are expressed by: EQU P.sub.1,P.sub.2 =2(Nup-Ndown)/(Nup+Ndown) (3)
where Nup is the number of up-spin electrons or the number of energy levels for an up-spin electron, and Ndown is the number of down-spin electrons or the number of energy levels for a down-spin electron.
The polarization factors P.sub.1 and P.sub.2 change depending upon the kind of ferromagnetic material, and some material takes a polarization factor of near 50%. In such a case, a resistance change rate of several tens % larger than a resistance change rate of the anisotropic magnetoresistive effects and the giant magnetoresistive effects can be expected.
A metal oxide is generally used as an insulating film of a ferromagnetic tunnel junction structure. This metal oxide is formed by depositing a metal layer and oxidizing the surface thereof through natural oxidation, plasma oxidation, thermal oxidation or the like. This insulating film forming method, however, may oxide only the surface layer of the metal layer and leave an unoxidized metal layer in the deepest region.
The unoxidized metal layer may form solid solution by reacting with the magnetic layer at a later high temperature process. In this case, a four-layer structure of magnetic layer/solid solution layer/insulating layer/magnetic layer or a five-layer structure of magnetic layer/solid solution layer/metal layer/insulating layer/magnetic layer is formed. Since solid solution is non-magnetic, electrons in the solid solution are not spin-polarized. Therefore, the polarization factor of an electron tunneling between the magnetic layers attenuates in the solid solution so that the resistance change rate given by the equation (2) lowers.